There are a number of techniques for coating substrates. For example, a substrate may be suspended above a cathode target of the coating material in a vacuum. Argon ions are caused to strike the target thereby knocking off small particles of the target material which adhere to the substrate. Such a technique is known as sputtering. Carbon can be sputtered but the deposition rate is low and the coating may be graphitic.
Coatings may be grown in a plasma. For example, a substrate may be used as a cathode in a D.C. or A.C. excited hydrocarbon plasma. Carbon ions are attracted to strike the substrate and form a carbon layer which, for appropriate temperature and pressure conditions, is diamond-like.
Techniques for growing carbon films are described in the following: Thin Film Solids 58 (1979) 101-105, 106, 107-116, 117-120, and in various British patents such as, British Patent Specification Nos. 2,047,877, 2,067,304, 2,069,008, 2,082,562 and 2,083,841.
Techniques for growing amorphous layers of silicon and germanium by glow discharge are described in the following article and its associated references Spear, W. E., Doped Amorphous Semiconductor, Advances in Physics, 1977, Vol. 26, No. 6, 811-845.
A disadvantage with the prior art glow discharge deposition technique is that in order to achieve reasonable deposition rates high potentials have to be applied to or developed at the cathode, so that the substrate is subjected to high energy incident particles. This tends to affect adversely the stress in the coating and increases the back sputtering rate. In some cases this prevents deposition of a layer at all.
British Patent Specification No. 2,069,009 describes a growth technique in which a substrate is arranged above an aluminum cathode. Carbon ions strike the cathode, some adhere to the cathode and some bounce off and stick onto the substrate. A bias voltage may be applied to the substrate. This layer has a lower strain than similar layers deposited on cathodes but it still contains undesired interstitial carbon.
Additional efforts to provide coatings on substrates are known from U.S. Pat. No. 4,400,410. U.S. Pat. No. 4,400,410 describes a system for depositing discrete layers of carbon, silicon, and germanium onto electrically insulating substrates in a D.C. glow discharge chamber. This uses separate supplies of a hydrocarbon, silane, and germane respectively. The disclosed process must operate at a temperature below 300.degree. C. to avoid forming an infra-red absorbing coating. The description makes no reference to strain-free coatings, or the hydrogen content of any coating.